The present invention relates in general to raster scan oscilloscopes and in particular to an oscilloscope employing pixel intensity gradation to provide smooth waveform displays.
Digital oscilloscopes typically display waveforms representing magnitudes of input signals as functions of time, the waveforms being formed by selectively illuminating pixels on a cathode ray tube (CRT) screen. Typically, an input signal is sampled and digitized to provide a waveform data sequence, each successive element of the sequence representing the magnitude of the waveform at a successively later time. In a raster scan oscilloscope, displays are formed on a screen by selectively illuminating pixels organized into an array of horizontal rows and vertical columns, each column representing a different time period and each row representing a different magnitude. The waveform data sequence is processed to provide a "bit map", a stored data array indicating whether or not each pixel is to be illuminated. In a horizontal raster scan oscilloscope, an electron beam periodically sweeps horizontally over each pixel row and the beam intensity is modulated during its sweep so that pixels to be included in a waveform are illuminated while pixels not included in a waveform are not illuminated. The determination as to whether each pixel is to be illuminated is made in accordance with the stored bit map.
Since there are a finite number of pixels on the screen, rapidly rising and falling portions of a waveform can appear discontinuous or jagged due to differences in elevation of horizontally contiguous pixels included in the waveform. In addition, due to small amounts of noise introduced by oscilloscope input circuitry, portions of the waveform can appear to oscillate between contiguous pixel rows when the magnitude of the input signal lies between values represented by the elevations of the two pixel rows. This oscillation may give the false impression that the waveform has a high frequency component. In addition, the vertical resolution of the display is limited by the number of rows of pixels on the screen.